Best match is a loose search. It will match related terms (like plurals, misspellings, etc.). When sorted, the other searches seem to force actual keyword results only (although there are still the odd results that can’t be explained).
So, if you wanted listings offering one or more Spider-Man mugs, then you should search something like:
(Spider-Man,Spiderman,"Spider man") (Mug,Mugs)
The first group catches common spellings of Spider-Man, and the second obviously ensures you are searching for either a single “Mug” or a collection of “Mugs”.
I would also refine that search to exclude decal stickers that show up in the results. E.g.:
(Spiderman,Spider-man,"Spider man") (Mug,Mugs) -(Decal,Sticker)
Note "Spider man" is quoted so it only matches that - without the quotes, something like a listing title of "A mug with a spider and a man on it" would be matched.
